Early epoch stellar positions from the Bordeaux Carte du Ciel


Michael Odenkirchen, Caroline Soubiran, J. François Le Campion, Bordeaux


Complete text (abstract only):

Astrometric measurements made by the satellite Hipparcos have brought forth a homogeneous system of precise proper motions of about 105 stars, which is unprecedented in its combination of quality and quantity. Such proper motions are of great importance in various sorts of kinematic studies. However, the proper motion system needs to be disseminated and extended to stars of fainter magnitudes by means of earth-bound observations. Here, photographic records of star positions from the beginning of the 20th century present a valuable and indispensable source of information. A first important step was made with the construction of the ACT Catalogue (Urban et al. 1997), which incorporates the ancient plate measurements of the so-called Astrographic Catalogue (AC). A second series of plates, the so-called Carte du Ciel (CdC), taken as part of the same survey project as the AC, has not been used systematically for astrometry so far. The advantage of the CdC is that it reaches about 3 mag fainter than the AC. It provides however less redundancy from plate overlap.

The Observatoire de Bordeaux has initiated a program for the digitisation and measurement of its collection of CdC plates which covers the zone of +11 to +17 declination and contains positions for about 8 × 105 stars at epochs mostly between 1910 and 1920. As a pilot study for this program, we are currently working on two regions near the galactic anticenter, one surrounding the open cluster NGC 2355, the other intersecting the galactic plane at l  195. These regions were selected for practical as well as for scientific reasons. The plates have been digitised on the MAMA machine at the Observatoire de Paris. Object detection was made using the software SExtractor (Bertin & Arnouts 1996). The plate limit proves to be at B  15 and a completeness of 80 % is reached up to 14th magnitude.The plates of the CdC provide three equal exposures with a small angular separation of  10". A loss of about 20 % of stars must be accepted due to interferences between the grid lines and the triple exposures. Refined object positions are derived by fitting a 2-D triple gaussian to the images (cf. Dick et al. 1993). A direct reduction of the measurements to the Hipparcos catalogue is not advisable because of the density of Hipparcos stars being low and because of problems from overexposure. Instead, the use of the ACT as a reference catalogue is appropriate. Rms residuals in the reduction to ACT are typically around 0.15". Comparing the positions of stars in the overlapping borders of adjacent plates, we obtain consistency on the level of 0.2" for non-corner regions and 0.3" for corner regions. This indicates typical positional accuracies of 0.15" and 0.2" respectively per star and plate.

Proper motions are determined in combination with recent observations obtained with the Bordeaux meridian circle (see Colin, this volume, p. 141). With an epoch difference of 80 years, a proper motion accuracy of typically 2.5 mas/y is achieved.



Bertin E., Arnouts S., 1996, A&AS 117, 393

Dick W. R., Tucholke H. J., Brosche P., Galas R., Geffert M., Guibert J., 1993, A&A 279, 267

Urban S.E., Corbin T.E., Wycoff G.L., 1997, ACT Reference Catalogue, US Naval Observatory, Washington D.C.

Bibliographical details:

Michael Odenkirchen, Caroline Soubiran, J. François Le Campion: Early epoch stellar positions from the Bordeaux Carte du Ciel. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae ; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 145-146.